The invention relates to the field of drug analytics and the detection and quantification of the main constituent of Mitragyna speciosa, commonly referred to as ‘Kratom’, mitragynine (systematic name: (E)-2-[(2S,3S,12bS)-3-ethyl-8-methoxy-1,2,3,4,6,7,12,12b-octahydroindolo[2,3-a]quinozilizin-2-yl]-3-methoxy propenoic acid methyl ester) and its principle metabolite 8-desmethylmitragynine (systematic name: (E)-2-[(2S,3S,12bS)-3-ethyl-8-hydroxy-1,2,3,4,6,7,12,12b-octahydroindolo[2,3-a]quinozilizin-2-yl]-3-methoxy propenoic acid methyl ester—also referred to in the literature as 5-desmethylmitragynine and 9-desmethylmitragynine). The recent increase in recreational drug-induced psychostimulation, and the concomitant variety and number of substances ingested to achieve this effect, includes the use of kratom. Kratom is purported to have medicinal properties and, although illegal in certain countries, it remains legal in the US, UK, and most of Europe. There have been recent reports of numerous fatalities associated with its ingestion and there is increasing interest in kratom detection, by way of mitragynine and its metabolites, for toxicological and research purposes. Described analytical methods for detection and quantification of mitragynine and metabolites use the relatively expensive, specialist operator-dependent techniques of high performance liquid chromatography (HPLC) or mass-spectroscopy linked to either gas chromatography (GC-MS) or liquid chromatography (LC-MS) (e.g. Kaewklum, 2005; Janchawee, 2007; Le, 2012). The techniques can be further complicated by requiring a sample pre-treatment prior to analysis. Thus, there is a clinical and forensic need to rapidly and economically detect and quantify mitragynine and its metabolites in patient samples and substances suspected of incorporating kratom.